1. Field of the Invention
The present invention relates to optical modulators that are of interest to communication systems, and more particularly, to resonant optical modulators used in such systems.
2. Discussion of the Related Art
As the demand for high-speed and complex optical communication systems continues to grow, so too has the need for reliable high-speed devices needed for modulating optical signals traversing such systems. Optical modulators are of great interest in operating a fiber optic communication system in the range of 2.5 to 10 Gbps (Giga bits per second), and potentially to 40 Gbps or more. Of particular interest are modulators having low operating voltage and low optical and/or electrical losses that can reliably modulate optical signals transmitted through optical fiber or other optical media. Also of interest are modulator devices that can be integrated into optical circuits that may comprise a plurality of modulators and other related devices disposed on a common substrate.
There exist certain types of anisotropic materials of uniaxial crystal whose permittivities are directly proportional to an applied electric field and vary almost linearly with an applied electric field. This electrooptic property is known as the Pockels effect. Applying an electric field across an area occupied by a light signal in these types of uniaxial materials can modulate the light signal utilizing the electrooptic properties of the material. Because wave velocity is generally inversely proportional to the square root of the permittivity of the material in which the wave is propagating, a change in permittivity affects wave velocity within the electric field. In uniaxial crystal waveguides, this effect is advantageously used to shift a phase of the carrier wave traveling through the crystal and thus modulates the carrier wave phase.
In a simple form, a phase modulator can consist of a single channel optical waveguide formed within uniaxial material with electrodes disposed in such a way that an electric field applied across the channel modulates the phase of the carrier wave propagating within the channel. Another commonly used waveguide structure used for optical modulation is the Mach-Zehnder Interferometer (MZI), as illustrated in FIG. 1. An MZI includes a waveguide channel 12 having two opposing Y junctions 10a and 10b joined by waveguide arms 12a and 12b. Waveguide 12 is formed within uniaxial material to exploit electrooptic effects, as described above. In the illustrated MZI, the waveguide junctions are symmetrical and operate as 50:50 power dividers.
FIG. 2 shows an optical modulator using an MZI having coplanar waveguide electrodes 22-26 formed over optical waveguide 12. Electrodes 22 and 24 are supplied with a ground potential, while electrode 26 is supplied with an RF signal that terminates at impedance RT. In operation, when a carrier wave from a light source, for example a DFB laser, enters at optical waveguide input 14, the carrier power is evenly split at the first Y junction 10a into the two light channels of the MZI arms 12a and 12b. By applying an electric field between the electrode 26 and ground electrodes 22 and 24, oppositely oriented electric field vectors exist in the crystal, one in each MZI arm 12a and 12b. Consequently, the carrier light wave within each of the arms is complementarily phase shifted relative to one another in push-pull fashion. Light from each arm is then combined at Y junction 10b where constructive or destructive interference resulting from combining phase shifted carrier waves causes signal intensity modulation. When the total phase shift xcex8 between the carrier waves in arms 12a and 12b is such that xcex8=xcfx80, light entering the device at 14 radiates into the substrate and results in zero channel output at 15.
Of uniaxial materials used to fabricate optical modulators, lithium niobate (LiNbO3) or lithium tantalate (LiTaO3) are popular substrate choices. LiNbO3 is widely used due to its combination of low loss characteristics, high electrooptic coefficients, and high optical transparency in the near infrared wavelengths used for telecommunications. Its high Curie temperature (1100xc2x0 C.-1180xc2x0 C.) makes it practical for fabrication of optical waveguides because strip waveguides can be fabricated by means of Ti-indiffusion at temperatures near 1000xc2x0 C.
LiNbO3 wafers are available in three different crystal cuts (x-, y-, and z-cut). FIGS. 3a and 3b respectively illustrate a cross-section of x-cut and z-cut LiNbO3 substrates 11. For the most pronounced electrooptic effect, the strongest component of the applied electric field is aligned with the z-axis of the crystal (because the z-axis has the highest electrooptic coefficient) to take advantage of the r33 coefficient. On z-cut LiNbO3, vertical fields are used with a TM mode to take advantage of the r33 coefficient. On x-cut, horizontal field electrodes and a TE mode utilize the r33 coefficient.
As shown in FIG. 3a, x-cut crystal substrates require placement of MZI arm 12a between electrodes 22 and 26, and arm 12b between electrodes 26 and 24. FIG. 3b illustrates a z-cut crystal, where RF and ground electrodes must be placed directly over waveguide arms 12a and 12b. Thus, in both the x- and z-cut cases, applied electric fields from respective TE and TM modes of the RF input are aligned with the z-axis of the LiNbO3 crystal. While not shown in FIG. 3b, an insulation buffer film such as silicon dioxide or Al2O3 may be used as a buffer to minimize z-cut LiNbO3 optical losses that occur through TM mode absorption in the electrode metal. Buffer films are also beneficial to x-cut LiNbO3 devices operating at high frequency.
LiNbO3 modulators are used external to a source of an optical signal, unlike directly modulating a light source that provides an optical signal, such as a laser diode. External modulation avoids chirping (a time-dependent fluctuation of the wavelength in a modulated optical beam) and patterning effects inherent to directly modulated lasers, which is particularly important in digital applications requiring large extinction ratios.
LiNbO3 modulators are widely used in digital applications to modulate a carrier wave using RF input in several modulation formats. Of particular interest are return-to zero (RZ) modulation formats. The RZ format has been employed in recent high-bandwidth terrestrial and submarine systems, especially those requiring long transmission distances. Dispersion managed soliton and other narrow-pulse transmission techniques can be considered specialized versions of RZ transmission.
Unlike the nonreturn-to-zero (NRZ) format, where binary data represented by a modulated carrier wave output maintains a high level when representing a xe2x80x9c1xe2x80x9d in a bit interval, in RZ coding of binary data, the output returns to a xe2x80x9czeroxe2x80x9d level for one or more portions of the bit interval. In the conventional NRZ pulse format, interaction between self-phase modulation (SPM) and group velocity dispersion (GVD) causes transfer of energy from the center of the pulse toward the pulse edges. Use of RZ format in a dispersion-managed system allows for balancing SPM and GVD, resulting in greater pulse-to-pulse consistency.
Recent high dense wavelength division multiplexed (DWDM) channel loading, increased bit-rate requirements of next-generation systems, and the desire to build wavelength-intelligent networks, have pushed the capabilities of the NRZ transmission to its limits. Thus, there remains a need in the art for external modulation devices capable of producing pulse forms necessary to transmit broad band optical signal data through optical fibers, and to alleviate the aforementioned problems associated with present optical communication systems.
The present invention has been made in view of the above circumstances and provides a resonant optical modulator for optical communication systems.
One aspect of the present invention relates to using a resonating electric field to modulate a light signal.
Another aspect of the present invention relates to modulating a light signal in an electrooptic substrate using a resonating ring circuit.
Yet another aspect of the present invention relates to using a coplanar resonant ring circuit to form a light pulse.
Still yet another aspect of the present invention relates to a modulator having a resonating ring electrode that is a pulse-forming generator in an optical communication system.
Another aspect of the invention relates to a pulse generating modulator that operates at a single modulating signal frequency.
Yet another aspect of the present invention relates to a resonant optical modulator having a resonant electrode formed into a closed loop that is capable of supporting all harmonic modes.
Another aspect of the present invention relates to a resonant optical modulator having a resonant electrode formed into a closed loop having one or more slit, notch and/or stub structures.
Still another aspect of the present invention relates to an optical modulator having low coupling losses and low drive power requirements.
Additional aspects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The aspects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.